I have been installing systems in my vehicles for over 10 years and have never had anything as strange as this happen...

I am running 4 8" Infinity Reference 860W subwoofers. Nothing about the setup changed except the new subs and new custom box. Amp is the same, deck is the same, wiring is the same.

Here is the bizarre happenings: When I'm driving down the road with my windows UP, the subs sound amazing! But as soon as I roll down my window even 1/4 of the way, the subs sound like they are distorting, making a sort of 'pop' noise. If I am parked and my window is down, they sound fine...

It also does this at low volume or high volume, so it's not an over-power to the sub problem.

So long story short, it seems to only happen when I am driving 30mph+ and have my window rolled down. The noise is definitely the subs, as it does the 'pop' on almost every note.

what youre most likely experiencing is a low frequency transfer function. Which is simply the overall effect that the environment has on the freqeuncy response and amplitudes of a sound pattern. The most noticeable time this happens is when you take a subwoofer out of the vehicle and put it on the floor. It is much louder inside of the vehicle because of the environment. All you can really do is try placing the enclosure in different places and firing in differnt directions if possible.

another possibilty is if your subwoofers are in a sealed enclosure it may not be air tight. with the windows up you are basically creating a sealed environment (although not an ideal one) for the subwoofer. and when the windows are down at higher speeds the air pressure of the vehicle changes and could effect how the subwoofers sound if not properly sealed.

Hmm...I didn't think there was enough pressure to affect the subs in quite a drastic way. If I were going down the highway at 70mph and had the window just right, then I could see that possibly happening...but you think it would do that even at 30mph? Thanks for the theories! It might be just that..

My first thought would be pressure change as well but at only 30mph i wouldn't imagine that to be enough to disrupt much with the amount of power behind them. My only other thought would be to check your custom box dimensions and the specs on your subs for how many cubic feet they require per sub. Too much or too little enclosure space combo'd with the pressure change might do it IMO

When you are driving with the window down the inside of the vehicle is in a low pressure state. If you have a soft top you could see the windows suck in with the windows even slightly cracked.
You want to verify this crack the windows slightly to create the crack sound then turn the blower on high and cool so outside are is forced into the cabin balancing the pressure back to zero or positive pressure.

Not sure on the pop noise. Possibly the new enclosure is air tight and with the pressure differential the sub cone is already out or even vibrating. Like a motor to a generator if you apply a voltage to a motor the shaft spins. If you spin the shaft with no voltage supplied you create a current. So in a speaker system, the speaker moves when an oscillating current passes through the magnet interacting with the cone causing it to vibrate. With a large enough cone and vibration you may be generating a voltage that once builds to a point it discharges causing the crack. As it builds it distorts the sound. Or possibly it is hitting its max outward position due to the offset fron the vac condition.

Is the enclosure fairly tight? Drill a hole in it to equalize the pressure. Or buy quieter tires.

It's clearly because the window is down and simple trouble shooting would tell you that. Nothing else changes when the window is up, compared to when the window is down except for the amount of pressure in the vehicle. Even at a dead stop the pressure is different. The air can escape easier from the window being down and drops the pressure of the vehicle.